A conventional Automatic Gain Control (AGC) circuit comprises an amplifier, with a gain controlled by a control signal, and a generator of this control signal. However, for the purpose of this description and a simplification of the nomenclature, the AGC term will be used here only for the amplifier controlled with a control signal, which is generated in a separate calibration block.
The elementary characteristic of an AGC circuit is a relation of its gain [dB] to the control signal [V]. This characteristic can be either linear or non-linear. Because of possible lack of precision during the production phase, characteristics of circuits of the same type can differ.
AGC circuits are commonly used in signal receivers, in order to set a required level of signal that is input to a signal-processing module. In receivers that perform frequency conversion (for example, demodulation), two AGC circuits are usually used—the first for control of the level of a high frequency input signal and the second for control of the level of a lower frequency input signal.
Two AGC circuits are also used when more precise control of a received signal is required or when one of the AGC circuits has insufficient regulation range.
Signal receivers often need to determine the power of the input signal. It can be calculated on the basis of the level of the output signal of the last AGC circuit and individual gains of successive AGC circuits.
However, because of production imprecision, characteristics of AGC circuits are deformed, and a measurement based on a general (theoretical) characteristic of a circuit is burdened with an error. This error can be minimized by calibrating all AGC circuits used in the receiver, to determine their characteristics precisely. The calibration process can be performed during production phase, where it is possible to connect test signals of a predetermined level to the receiver. Obviously, the time of calibration influences the production efficiency and the final cost of the device. Therefore, the calibration should be as simple and fast as possible.
An example of a signal receiver, which requires such calibration, is a cable modem. The structure of a conventional cable modem is presented in FIG. 1. A high frequency (radio frequency) input signal S_RF is received by the tuner 101, for example a Microtune MT2040 chip, which converts the signal to an intermediate frequency signal S_IF. Next, the signal is passed through a SAW filter 102, for example with the center frequency of 43.75 MHz. Next, it is input to an intermediate frequency amplifier 103, for example a Microtune MT1233 chip. Its output signal S_IF is connected to a processor (which may be a demodulator) 104 of the cable modem, for example an ST Microelectronics STV0396 chip. The tuner 101 gain is controlled with the AGC_RF signal, while the gain of the IF amplifier 103 is controlled with the AGC_IF signal.
A commonly used specification, which defines the functionality of a cable modem, is DOCSIS (Data-Over-Cable Service Interface Specification). DOCSIS version 1.1 requires that a cable modem should be able to determine the level of the received signal with the accuracy of +/−3 dB. Such a high level of accuracy requires the modem to be calibrated, in order to accurately and precisely determine the characteristics of AGC circuits.
A typical method of cable modem calibration is presented in the PCT application publication No. WO 01/24469, entitled “Method and system for estimating input power in a cable modem network.” The calibration involves inputting signals of various levels and frequencies and storing their corresponding AGC values. Such a method of calibration is ineffective, since it is performed fox 12 signal levels only, while the pain for the remaining values can be calculated on the basis of the measured neighboring values. In addition, the method is suitable to a single AGC circuit only, while the method of the present invention is applicable to devices with two or more AGO circuits.